1. Field of the Invention
The invention relates to a device for scanning a record carrier which comprises essentially parallel tracks, the device including a read head for scanning the record carrier, during which operation a scanning point is moved across the record carrier, the read head including a detector for producing detection signals which depend on the position of the scanning point relative to the track. A first signal processor is coupled to receive the detection signal from the read head and derives a tracking error signal and a track position signal, the tracking error signal being indicative of the deviation between the scanning point and the middle of the nearest track over a certain range on either side of each track, and the track position signal indicating whether the scanning point is positioned between two tracks or essentially on a track. The device further includes first comparing means for detecting whether the track position signal exceeds a first threshold, and second comparing means for detecting whether the tracking error signal is situated within an amplitude window established by a second positive threshold and a third negative threshold. A second signal processor derives a track loss signal from the results of the detections performed by the first and second comparing means, which track loss signal denotes that the scanning point no longer follows the track.
2. Description of the Related Art
A device of this type is known from Published International Patent Application No. WO 91/05340. That Application describes an optical scanning device in which the track loss signal is produced when the track position signal, in the form of the envelope of the r.f. read signal, intersects a predetermined threshold and, simultaneously, the tracking error signal is situated outside of the amplitude window.
Although the track loss signal is reliably derived in such prior-art device, it may be erroneously produced in the case of disc errors causing narrow interruptions of the information layer (also known by the German term of "Keil"). A "Keil" may result in a brief disturbance of the detected signals causing substantially simultaneously, the tracking error signal to be outside of the amplitude window and the track position signal intersect the first threshold. Consequently, an erroneous track loss signal is generated.